The present invention is a process for the cleaning of the inner surfaces of a chemical vapor deposition reactor used in the production of polycrystalline silicon. The process comprises impacting the surfaces to be cleaned with solid carbon dioxide pellets. The carbon dioxide pellets dislodge silicon deposits from the surface of the reactor without damaging the surface of the reactor and without providing a source for contamination of polycrystalline silicon produced in the cleaned reactor. The present process is particularly useful for the cleaning of the inner surfaces of chemical vapor deposition reactors used in the production of semi-conductor grade silicon.
As the density of circuits formed on semi-conductor silicon chips continues to increase, there is an ever increasing need to provide semi-conductor grade silicon of greater purity. In general, it is necessary to provide semi-conductor grade silicon that has impurities such as phosphorous, boron, and carbon reduced to the parts per billion range. Typically, as a first step to producing this ultra-high purity semi-conductor grade silicon, a silane (SiH.sub.4) or a halosilane such as trichlorosilane is decomposed onto a heated silicon element within a chemical vapor deposition (CVD) reactor. The polycrystalline silicon formed by this process is subsequently converted to monocrystalline silicon by standard techniques.
During the CVD process, amorphous silicon dust may be formed in the reactor by a process commonly referred to as homogeneous nucleation. In addition, deposition of silicon on the internal surfaces of the reactor can occur. Silicon deposits on the internal surfaces of the reactor act as an insulator causing the reactor bulk gas temperature to increase. A consequence of this reactor bulk gas temperature increase is that homogeneous nucleation, creating silicon dust, increases. This silicon dust typically contains high levels of contaminates and can settle on the product polycrystalline silicon causing unacceptable surface defects and contamination.
To prevent these problems with the silicon dust, it is often necessary to reduce the power to the forming silicon elements. While reducing power to the reactor is effective in reducing the temperature within the reactor, the reduction in power also reduces the rate of silicon deposition and the overall yield of the process per unit time. In addition, if silicon build up on the surfaces of the CVD reactor becomes too great, particles of silicon my dislodge and fall onto the forming polycrystalline silicon elements causing contamination and unacceptable surface imperfections.
Therefore, it is an objective of the present invention to provide a process for removing buildups on the inside surfaces of a CVD reactor used in the production of semi-conductor grade polycrystalline silicon. It is a further objective of the present invention to provide a process for cleaning the inner surfaces of the CVD reactor that does not pit, mar, or scour the surface being cleaned, since this could expose impurities in the reactor materials of construction and negatively impact the quality of product silicon. Finally, it is an objective of the present invention to provide a cleaning process that does not impart contamination to the CVD reactor that could subsequently contaminate product silicon.